Influx of Ca2+ through the NMDA subtype of glutamate receptor is widely accepted as a trigger for many forms of neural plasticity. However, direct support for this model has been elusive, since indirect activation of dendritic voltage-sensitive Ca2+ channels is difficult to exclude. We have optically measured synaptically induced changes in cytoplasmic free Ca2+ concentration in pyramidal cell dendrites in hippocampal slices. Steady postsynaptic depolarization to the synaptic reversal potential eliminated the effect of voltage-sensitive Ca2+ channels. Under these conditions, synaptically induced Ca2+ transients were observed, which were blocked by the NMDA receptor antagonist APV. In addition, the magnitude of LTP was diminished when induced with the postsynaptic membrane held at progressively more positive potentials. LTP could be completely suppressed at potentials near +100 mV. These results provide important experimental support for a role for Ca2+ influx through NMDA receptors in synaptic plasticity.